Optical chip and method for producing an optical chip having a reinforced structure

ABSTRACT

The invention relates to an optical chip and to a method for producing an optical chip having a reinforced structure. The chip has a substrate, optical waveguides arranged on the surface of said substrate, and at least one optical structure for influencing the optical properties of the optical waveguides, and an interconnected laminar reinforcing or stiffening structure constructed in the form of a cross which is arranged centrally on the substrate with the provision of diametrically opposite cut-outs.

FIELD OF THE INVENTION

The invention relates to an optical conductor component in the form ofan optical chip to which it is possible to connect optical conductors(LWL), such as, for example, individual fibres or fibre arrays and/orother optical conductor components such as photodetectors orphotodetector arrays. The invention further relates to a method forproducing such an optical chip.

BACKGROUND OF THE INVENTION

In order to be able to implement an optical coupling between planar LWLcomponents, that is to say optical chips, and assemblies to be connectedthereto, it is necessary for the optical chips to be strengthenedstructurally so that they withstand mechanical loads associated with theconnection of optical conductors and other LWL assemblies. For thispurpose, the chips are usually constructed in a thickened fashion byproviding the entire underside of the respective chip with an additionalglass plate which is pressed onto the chip after adhesive material hasbeen interposed. The glass plate stiffens the chip structurally to asufficient extent such that the latter withstands the mechanical loadsto be expected.

Simply pressing the glass sheet onto the chip has, however, thedisadvantage that the chip, which has a slightly curved shape because ofits customary production process, experiences concave bending andbending back. The latter leads to strains in the chip, and thus toinfluences exerted on the optical conductors, made from glass,accommodated in the chip, since additional instances of refraction arecaused in the optical conductors because of the strains. Furthermore,particularly in the case of optical chips with thermooptical structures,which are necessary, for example, for attenuating or for switching, theglass plate applied over the entire surface of the underside isdisadvantageous in thereby worsening the good thermal conduction whichis required in the reverse direction owing to the silicon or glasssubstrate of the chip.

SUMMARY OF THE INVENTION

The invention creates an optical chip, with a thermooptical structure,which has adequate structural strength, and in the case of which thereis lessening of the impairments, caused by strength-enhancingcomponents, of its optical, in particular thermooptical properties.Furthermore, the invention creates a method for producing an opticalchip with the aid of which the optical chip can be produced withimproved optical properties, and yet with an adequate structuralstrength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of an optical chip according to anembodiment of the invention in a view from below,

FIG. 2 shows a schematic illustration of an optical chip according toanother embodiment of the invention in a view from below, and

FIG. 3 shows a cross section through an optical chip according to anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The optical chip has a substrate after an aspect of the invention, onthe top side of the substrate arranged fiber-optic cables and at leastan thermaloptical structure located on the top side of the substrate forthe influence of the optical characteristics of the fiber-optic cables.Furthermore the optical chip exhibits a coherent, layer shapedreinforcement structure, which is appropriate on the lower surface ofthe substrate in accordance with the invention. The reinforcementstructure and the respective thermaloptical structure are in such amanner intended that the reinforcement structure at least partly extendsunder going around the respective thermaloptical structure in the planview of the chip seen in a distance from the respective thermalopticalstructure crosswise over the substrate and/or along the free edge of thesubstrate around the respective thermaloptical structure.

The term “cohesive” stiffening structure is understood in this case tomean that the stiffening structure is not provided in a fashionconsisting of separate individual elements, but that it extendscontinuously, that is to say without a break, on the substrate.

Because the thermooptical structures are left free from the stiffeningstructure, backward heat removal, that is to say downward heat removal,is not prevented below these structures. Consequently, it is possible toset up between the top and undersides of the chip an adequately steeptemperature gradient which is essential for effective functioning of thethermooptical structures. An overall improvement in thermoopticalproperties of the chip is thereby achieved. Although provided as alayer, that is to say in a planar fashion, the stiffening structure hasthe function of a stiffening rib arrangement with the aid of which anadequate structural strength of the substrate is achieved. The strainsin the substrate are reduced on those sections of the substrate whichare left free from the stiffening structure, since the stiffeningstructure in these sections cannot force the substrate directly to adaptto its own shape. An improvement in the optical properties of the chipis achieved thereby. Optical couplings of LWL lines external to the chipand of assemblies can be arranged on the regions of the chipstrengthened by the stiffening structure.

After a further aspect of the invention the optical chip has asubstrate, on the top side of the substrate arranged fiber-optic cablesand at least an optical structure located on the top side of thesubstrate for the influence of the optical characteristics of thefiber-optic cables. Furthermore the optical chip exhibits a coherent,layer shaped reinforcement structure, which is appropriate on the lowersurface of the substrate. The connection between the reinforcementstructure and the substrate is without tension intended. According tothe embodiment, the stiffening structure is almost excluded frominfluencing the optical properties of the chip, because the substrate isnot necessarily adapted to the shape of the stiffening structure, as aresult of which strains would otherwise, as explained above, be producedin the substrate and the optical conductors arranged thereon. Theexplained stress-free connection between the substrate and stiffeningstructure is advantageous both in the case of optical structuresprovided as thermooptical structures, and in the case of the use ofother optical structures such as AWG (Arrayed Waveguide Grating) orsplitter structures. In the case of the use of non-thermoopticalstructures on the optical chip, the latter is likewise preferablyprovided with the stiffening structure not over the entire area but, inparticular, with leaving free of the optical structures.

The production of an unstressed connection between substrate andreinforcement structure effected favourable-proves as in the followingexample.

In accordance with the invention a preferential procedure formanufacturing a structure-strengthened optical chip with a substrate andon its top side arranged fiber-optic cables, with one like describedstructure planned, is preceding with which a hardenable bonding agent isapplied layer shaped on the lower surface of the substrate, a layershaped reinforcement structure on the bonding agent is in particularpresented, and the bonding agent under training an unstressed connectionbetween the reinforcement structure and the substrate without additionalmechanical effects on the substrate and the reinforcement structure ishardened. It is to be noted in this case that the adhesive can also theapplied in a layered fashion to the underside of the substrate by virtueof the fact that it is applied in a layered fashion to the side of thestiffening structure facing this underside, the stiffening structurethen being laid onto the substrate.

Because the stiffening structure is merely laid onto the substrate, andthe adhesive introduced between the substrate and the stiffeningstructure is cured simply without further external mechanical action onthe substrate and the stiffening structure, a stress-free connection isachieved between substrate and stiffening structure which is attended bythe above named improvements with regard to the optical properties ofthe chip.

Consideration is given as adhesive to glues, in particular glues bymeans of which a satisfactory bonding of the stiffening structure on thesubstrate is achieved without excessive supply of heat.

The chip can be ground and polished at the circumference after thecuring of the adhesive and the possible application of additional layersand/or assemblies to be applied to the top side of the substrate.

In the case when thermooptical structures are provided on the opticalchip, according to the invention these structures are left freeextensively from the stiffening structure, seen in the chip plane. Inthe case of other optical structures such as the above named splittersand AWG structures, the stiffening structure can admittedly be appliedto the entire lower lateral surface of the substrate of the opticalchip, because there is no need to form a temperature gradient from topto bottom; however, it is preferably provided likewise only partially,in particular with leaving free of the optical structures, because theoptical properties of the chip are thereby improved.

According to an embodiment of the invention, the stiffening structure isconstructed with a central cut out in the form of a frame runningpartially around the edge of the substrate. The stiffening frame therebyachieved and arranged on the circumference of the chip results in astrong stiffening effect which permits the central cut out to beconstructed with large dimensions in order for a plurality of differentthermooptical structures to be arranged there. This stiffening structureis suitable, in particular, for rectangular chips, the thermoopticalstructures preferably being arranged along a longitudinal side of thechip.

According to another embodiment of the invention, the stiffeningstructure is constructed in the form of a cross which is arrangedcentrally on the substrate with the provision of diametrically oppositecut-outs. This stiffening structure is suitable, in particular forcircular chips. The bars of the cross of the stiffening structurepreferably extend at least up to the lateral edge of the substrate suchthat optical lines such as, for example, glass fibres, can be led up tothe chip from the side thereof and can be fitted there. The fourcut-outs produced are embraced thereby respectively on two sides of thestiffening structure in such a way that the circularly arcuate edgesides of the substrate, which are assigned to the cut-outs, remain free.

In order to prevent strains of the substrate occurring owing totemperature fluctuations during operation, the stiffening structure isadvantageously made from a material which has substantially the samecoefficient of thermal expansion as the substrate material. Differencesof the order of magnitude of 10⁻⁶ are to be understood in this case by“substantially”.

Glass is a suitable substrate material. Glass material functionsadequately in stiffening and can be connected simply to the substrate,which, for its part, is made either from silicon, or likewise fromglass, in particular by being glued. However, other materials with anadequate stiffening function and which can be handled well also comeinto consideration.

After a further aspect of the invention an optical chip is intended,with a substrate, on the top side of the substrate arranged fiber-opticcables, which are intended in a carrier layer located on the top side ofthe substrate, at least an thermaloptical structure located on the topside of the substrate for the influence of the optical characteristicsof the fiber-optic cables, and one on the top side of the carrier layertrained, layer shaped protection structure, over on the same trainedfiber-optic cables at least at the edge sections of the optical chipintended is seen in a distance from the respective thermalopticalstructure extended and in the plan view of the chip.

As thereby according to invention the advantage is described aboveobtained that in the range of the thermaloptical structure a strongtemperature gradient can be formed also from above downward by the chipsubstrate through sloping temperature, whereby the function of thethermaloptical structure is substantially improved. In this case, aprotective structure constructed directly above the thermoopticalstructure, which is likewise preferably provided in the carrier layer,would lead to thermal conduction via direct contact with the carrierlayer, such that heat would be conducted away upwards directly from thethermooptical structure, and this would lead to a reduction in thetemperature immediately at the thermooptical structure, and thus to areduction in the temperature gradient downwards. Given that, inaccordance with the invention, the thermooptical structure is left freefrom the protective structure on the top side of the carrier layer, sucha direct quick removal of heat does not take place, the heat transferbetween the carrier layer and the air surrounding the latter beingslight.

After a further aspect of the invention an optical chip is intended,with a substrate, on the top side of the substrate arranged fiber-opticcables, which are intended in a carrier layer located on the top side ofthe substrate, at least an optical structure located on the top side ofthe substrate for the influence of the optical characteristics of thefiber-optic cables, and, a layer shaped protection structure trained onthe top side of the carrier layer, which are intended over thefiber-optic cables planned on the substrate at least at the edgesections of the optical chip and which is without tension connected withthe carrier layer.

The stress-free connection between the carrier layer and the protectionstructure is made according to the unstressed connection between thereinforcement structure and the substrate, by a bonding agent on thecarrier layer or on this turned side of the protection structure layershaped applied, which is presented protection structure withbetween-lying bonding agent on the carrier layer and which is hardenedbonding agents without practice of external mechanical loads on thechip, for example by supply of warmth. With the unstressed connectionbetween protection structure and carrier layer the same favourableeffects are obtained, planned as with the unstressed connection betweenthe substrate and on its lower surface the reinforcement structure.

The respective aspects can as a function of the fact whether the chipwith thermaloptical structures or is not provided, or whether aprotection structure or a reinforcement structure for necessary isjudged, for example as a function of the respective inherent strength ofthe substrate and/or the extent of the sharpening procedure, as requiredin arbitrary combination or to be used individually. Preferentially bothprotection structure and stiffening structure are intended incombination.

The protective structure and the carrier layer are preferably made froma material which has essentially the same coefficient of expansion asthe substrate material.

In accordance with the preferred material for the stiffening structure,glass is a preferred material for the protective structure. Theprotective structure need not be provided cohesively, but can bearranged split up into mutually separate parts only in those regionswhere grinding and polishing of the optical conductor structures, thatis to say the optical conductor, is required in order to prevent themfrom being damaged, for example in the form of glass chippings.Consequently, the protective structure is provided directly below theoptical conductor structures, at least at the edge sections of the chip,since external optical conductor fibres are connected at the edge of thechip to its optical conductor structures, the latter being processed bygrinding and polishing the end face of the chip in order to constructoptical connecting points on the edge side.

Preferred embodiments of the invention are explained below withreference to the drawing, in which:

FIG. 1 shows a schematic illustration of an optical chip according to anembodiment of the invention in a view from below,

FIG. 2 shows a schematic illustration of an optical chip according toanother embodiment of the invention in a view from below, and

FIG. 3 shows a cross section through an optical chip according to anembodiment of the invention.

The optical chip 1 visible from FIG. 1 has a substrate 1′ (illustratedby the thick, rectangular line) in the form of a rectangle withdifferent side lengths. The substrate 1′ is provided on its underside 2with a stiffening structure 3 which extends along the two short edgesides 4, 5 and along one 6 of the two long edge sides (the top edge sidein FIG. 1). The stiffening structure 3 thereby forms a U-shapedstiffening frame which runs round along three edge sides 4, 5 and 6, andthereby is a partially embracing frame. At the two longitudinal ends ofthe chip 1, the two limbs 7, 8 of the frame-shaped stiffening structure3 thereby project a little, in the direction of the width of said chip,beyond the rectangular substrate 1′ thereof (illustrated by the thickline). According to this embodiment, the width of the frame-shapedstiffening structure 3 is just half as large as the width of thesubstrate 1′ of the chip 1. In the case when the chip 1 has largerdimensions or smaller dimensions as regards its length and width, thewidth of the frame-shaped stiffening structure can therefore be selectedto be larger or smaller than the width of the substrate. Provided on thecircumference of the chip 1 are four optical connecting points 9, 10,11, 12 which are respectively arranged at points on the chip 1 where thestiffening structure 3 extends. External optical conductor lines and/orother optical conductor assemblies can be connected to the chip 1 viathese optical connecting points 9, 10, 11, 12. The stiffening structure3 here constructs adequately large and stable connecting points as aresult of which it is possible, for example, to bond the externalcomponents, such as lines, to the chip 1.

The middle region 13 of the chip 1 is extensively left free from thestiffening structure 3 because the latter runs around partially on theedge side. The chip 1 is provided with a thermooptical structure 14 inthis middle region 13 (indicated diagrammatically by an ellipse) whoseterminals 15 are provided on the edge side of the free longitudinal sideof the chip 1. Very fine lead wires, for example gold wires, areconnected to these terminals 15, and so structural strengthening by thestiffening structure 3 is not required in this region.

Furthermore, chip-internal optical conductors in the form of opticalconductor structures 16 are arranged on the chip 1 and connect theoptical connections 9, 10, 11, 12 to one another and to thethermooptical structure 14.

Illustrated in FIG. 2 is an optical chip 100 according to anotherembodiment of the invention. This optical chip 100 has a circularsubstrate 100′ (illustrated by the thick line). On its underside 102,the optical chip 100 is provided with a stiffening structure 103 whichis structured in the form of a cross arranged centrally on the underside102 of the substrate 100′. The bars 104, 105 of the cruciform stiffeningstructure 103 cross one another at an angle of 90° and extendtransversely over the chip 1 up to its circumferential edge. The widthof the bars 104, 105 of the cruciform stiffening structure 103 isapproximately ¼ of the diameter of the chip 100.

Seen in the top view of the chip 100, four areas are left free thereonfrom the stiffening structure 103 because of the cruciform shape of thestiffening structure 103. In each of these four areas, there is providedon the chip 1 a thermooptical structure 106, 107, 108, 109 by means ofwhich the optical properties of the optical conductors provided on thechip 1, and thus the optical properties of the chip 1, can be set.

Coupling devices 114, 115, 116, 117 for connecting external opticalconductor lines and other optical conductor assemblies are provided ineach case at the four frontal end sections 110, 111, 112, 113 of thestiffening structure 103.

The wide extending stiffening structure 103 here offers sufficient areafor fastening the external assemblies on the chip 1 without any problem,for example by means of glueing. The stiffening structure 103 extends ata distance around the thermooptical structures 106, 107, 108, 109 at aspacing which is so wide that heat conduction downwards starting fromthe respective thermooptical structure 106, 107, 108, 109 is notprevented by an insulating effect of the stiffening structure 103.

A schematic sectional view of an optical chip 1, 100 according to theinvention is illustrated in FIG. 3. This sectional view relates both tothe optical chi) 1 according to FIG. 1 and to that according to FIG. 2,and so corresponding reference numerals are used for the respectivelyidentical assemblies.

As may be seen from FIG. 3, a substrate 1′, 100′ of the chip 1, 100 isconstructed in the form of a silicon layer. Arranged on the top side ofthe substrate 1′, 100′ is a carrier layer in the form of a glass layer17 (silicon dioxide), which extends over the entire substrate 1′, 100′and is of thinner construction than the substrate 1′, 100′(approximately 1/10 of the substrate thickness). Embedded in the carrierlayer 17 formed by the glass layer are optical conductor structures 18,that is to say optical conductors which likewise consist of glassmaterial. A thermooptical waveguide structure 14, 106, 107, 108, 109 isembedded in the carrier layer 17 of the chip 1, 100 on the right-handside in FIG. 3.

Arranged on the underside of the substrate 1′, 100′ of the chip 1, 100is a stiffening or supportive structure 3, 103 which is constructed inthe form of a layer made from silicon dioxide, which has a greaterthickness than the substrate 1′, 100′. As explained above, thesupportive structure 3, 103 does not extend over the entire substrate 1′100′. Instead of this, with reference to the chip plane, the location ofthe thermooptical waveguide structure 14, 106, 107, 108, 109 is leftfree from the stiffening structure 3, 103 arranged on the underside ofthe substrate 1′, 100′, that is to say no section of the stiffeningstructure 3, 103 runs below the thermooptical structure 14, 106, 107,108, 109. Consequently, heat can be eliminated without hindrance throughthe substrate 1′, 100′ in a downward direction (rearward direction)starting from the thermooptical structure 14, 106, 107, 108, 109.

Furthermore, a layered protective structure 19 in the form of severalmutually separate glass layers provided as cover plates or coverplatelets (only one being illustrated in the section) is arranged on thetop side of the glass layer 17 and has a greater thickness thanpreferably 10× the thickness of, the carrier layer 17 situatedtherebelow. Seen in the top view of the chip 1, 100, the protectivestructure 19 extends at a substantial spacing from the thermoopticalstructures 14, 106, 107, 108, 109. The protective structure 19 can alsobe provided cohesively. In order to protect the sensitive opticalconductor structures 18, it is arranged extending exactly over thelatter. In this case, it extends, in particular, at those points atwhich optical lines such as external glass fibres are led un andconnected to the chip 1, 100, in order there to provide protection forthe very sensitive glass terminal structures constructed on the chip 1,100, including from the top side of the chip 1, 100. That is to say, theterminal structures are packed in a sandwich-manner between thestiffening structure 3, 103 and the protective structure 19.Particularly on the end face of the chip 1, 100, the layered protectivestructure 19 arranged on the carrier layer 17 forms a protection for theoptical conductors 18 arranged beneath it, since in the event ofgrinding and polishing processes required on the end face, it preventsglass breakages at the sensitive waveguide structures. In accordancewith the production methods according to the invention, it is provided,like the stiffening structure 3, 103, as well, with an adhesive on itsside facing the substrate 1′ 100′ and is laid onto the glass layer 17.The adhesive is subsequently allowed to cure without, apart from theweight force, the exertion of an additional external mechanical load onthe glass layer 19 or another part of the chip 1, 100. The fact that theglass layer extends around the thermooptical structures provided, ifappropriate, on the chip 1, 100, prevents the undesired upward heatelimination (because of the build up of as steep a temperature gradientas possible with temperature dropping in the direction from tog tobottom) from being strengthened by direct thermal conduction, startingfrom the thermooptical structures 14, 106, 107, 108, 109, through afurther layer arranged directly over said layers on the carrier layer17. However, in the case of the use of non-thermooptical structures theprotective structure 19 can also extend over the latter.

1. An optical chip having a substrate, optical conductors arranged onthe top side of the substrate, at least one thermooptical structure,arranged on the top side of the substrate, for the purpose ofinfluencing the optical properties of the optical conductors, and acohesive, layered stiffening structure provided on the underside of thesubstrate, the stiffening structure and the respective thermoopticalstructure being provided in such a way that, seen in the top view of thechip, the stiffening structure extends in two directions perpendicularto each other at a distance from the respective thermooptical structuretransversely over the substrate and/or along the free edge of thesubstrate, at least partially around the respective thermoopticalstructure, wherein said stiffening structure is constructed in the formof a cross which is arranged centrally on the substrate with theprovision of diametrically opposite cut-outs.
 2. The optical chipaccording to claim 1, in which the stiffening structure is constructedwith a central cut-out in the form of a frame running partially aroundthe edge of the substrate.
 3. The optical chip according to claim 1, thestiffening structure being connected to the substrate in a fashion freeof stress.
 4. The optical chip according to claim 1, in which thestiffening structure is made from a material which has substantially thesame coefficient of expansion as the substrate material.
 5. The opticalchip according to claim 1, in which the stiffening structure is madefrom a glass material.
 6. The optical chip according to claim 1, inwhich the optical conductors are provided in a carrier layer arranged onthe top side of the substrate and in which a layered protectivestructure is formed on the top side of the carrier layer and is providedover the optical conductors at least at the edge sections of the opticalchip, and, seen in the top view of the chip, extends at a distance fromthe respective thermooptical structure.
 7. The optical chip according toclaim 6, in which the layered protective structure is connected to thecarrier layer in a fashion free of stress.
 8. The optical chip accordingto claim 7, in which the layered protective structure and the carrierlayer are made from a material which has essentially the samecoefficient of expansion as the substrate material.
 9. The optical chipaccording to o claim 8, in which the layered protective structure is aglass layer.